It is a part of the production process of building automobiles to examine the bodies for breaks in seals where parts are joined together that allow gases (e.g. exhaust fumes) or liquids (e.g. rain water) to leak into the interior compartments. Testing for the presence of such leaks has been accomplished by spraying water on the bodies and observing where water seeps through. An alternative method has employed emitting sound on one side of the compartment wall and listening with a microphone on the other side for where the sound may leak through.
In using sound to detect leaks, the system has to contend with ambient noises of the factory, alternate sound paths (such as through the windows), transmission of sound through the compartment wall, and indirect leakage paths (where the sound leaks into a channel, travels within the channel and exits into the compartment). These many paths that the sound travels is referred to as the multipath problem. By sinusoidally frequency modulating (FM) the sound, attempts were made to prevent the interfering multipath energy from nulling out the signal, but this was only effective on closed cavity applications. In open cavities such as an automobile chassis, energy escapes through the window areas, reflects off surrounding structures and interferes with the primary path energy with small attenuation.
Typical frequencies employed in the prior art were in the 35-40 KHZ range which enabled substantial transmission of energy through the compartment walls. Also, at these operating frequencies, the factory noise inband interference had been substantial. Setting the systems up consisted of manually adjusting detection thresholds to bring false alarms to acceptable levels. No range discrimination was employed.